The present invention relates to a lubricating oil for compressors such as those of automotive air conditioners, refrigerators, room air conditioners, and large-scale refrigerators for industrial use, and is intended to provide a low-viscosity lubricating oil for refrigerators comprising a branched-chain carboxylic acid monoalkyl ester with outstanding hydrolytic stability.
Unlike the properties required of ordinary lubricating oils such as engine oils, transmission oils, hydraulic oils and turbine oils, considerably different performance is required of oils used in the compressors of refrigerators because of the use of refrigerants in refrigerators. Particularly, in cases where esters are used as the lubricating oil for refrigerators, the requisite properties include lubricity as well as miscibility with refrigerants, electrical insulating property and a high level of hydrolytic stability.
Conventionally used refrigerants for refrigerators are fluorocarbons, such as those containing one or more chlorine atoms within the molecule, such as CFC-11 (trichloromonofluoromethane), CFC-12 (dichlorodifluoromethane) and HCFC-22 (monochlorodifluoromethane). However, it has become increasingly evident that such volatile substances are destroying the ozone layer and are a source of global warming, leading to various adverse effects on humans and the earth""s environment, so that the manufacture of CFC""s is now prohibited, and there is an international commitment to step-wise reduction in HCFC level. Natural refrigerants such as hydrocarbons, carbon dioxide and ammonia, and replacement fluorocarbons such as HFC-134a, are increasingly being used instead as chorine-free refrigerants that will avoid destroying the ozone layer.
Because of good miscibility that conventional chlorofluorocarbons have with lubricating oils based on mineral oils, such mineral oil lubricating oils were used as lubricating oils for refrigerators. Lubricating oils for refrigerators circulate in the refrigerator system along with refrigerant discharged from the compressor, and then the good miscibility between chlorofluorocarbons and mineral oil-based lubricating oils prevented the refrigerant and lubricating oil from separating.
However, replacement fluorocarbons (hydrofluorocarbons) such as HFC-134a have much higher molecular polarity than chlorofluorocarbons, they have poor miscibility with mineral oil-based lubricating oils. Poor miscibility between refrigerants and lubricating oils can result in the separation of the refrigerant and lubricating oil during operation, leading to seizure as a result of insufficiently lubricated compressors.
This has led to the use of polyglycols (polyalkylene glycols, polyethers) or esters, which have good miscibility, as lubricating oils to be used with replacement fluorocarbons. However, the electrical insulating property of polyglycols is a problem, and although they can be used in automotive air conditioners, they are unsuitable for use in hermetic refrigerator compressors.
Because esters have good electrical insulating properties, they can be used in various applications. Particularly, esters of neopentyl polyol and monovalent carboxylic acids (polyol esters) are actually being used.
However, among the polyol esters, esters of straight-chain carboxylic acids have poor hydrolytic stability and are decomposed through hydrolysis, and therefore polyol esters of mixtures of branched and straight-chain carboxylic acids, or polyol esters of branched-chain carboxylic acids are being used. Even so, the hydrolytic stability thereof is still poor, requiring complicated operations, such as the need to monitor air and moisture during assembly and maintenance or repair of refrigerators. Thus, there is a need to develop esters with better hydrolytic stability. There is a particular need to develop esters having better electric insulating properties and miscibility with refrigerants, and better hydrolytic stability.
On the other hand, aliphatic carboxylic acid monoesters such as coconut oil methyl ester, 2-ethylhexyl laurate, 2-ethylhexyl palmitate and isobutyl stearate have conventionally been used as base oils in metal processing oils and the like. However, such aliphatic straight-chain carboxylic acid monoesters suffer from poor hydrolytic stability, and have poor miscibility with replacement fluorocarbons, making them unusable as lubricating oils for refrigerators.
Furthermore, in light of the recent problem of global warming, there has been development of high-efficiency equipment for the purposes of energy conservation in various types of refrigerators in order to reduce carbon dioxide emission, which is a major cause of warming. To minimize energy loss in such equipment, there have been improvements in the materials for the parts that are used, and it has become essential to lower the viscosity of lubricating oils to reduce friction loss during operation. Examples of such low-viscosity oils include those with a kinematic viscosity (at 40xc2x0 C.) of 32 mm2/s (VG32), 15 mm2/s (VG15), 10 mm2/s (VG10), and some with even lower viscosity properties. There is thus increasing demand for esters with lower viscosity and better hydrolytic stability.
In view of the foregoing, an object of the present invention is to provide a lubricating oil for refrigerators that overcomes such drawbacks. A particular object of the present invention is to provide a low-viscosity lubricating oil for refrigerators with better hydrolytic stability.
As a result of extensive research to achieve the aforementioned object, the present inventors has found that certain aliphatic branched-chain carboxylic acid monoalkyl esters, prepared by the esterification of a specific aliphatic saturated branched-chain carboxylic acid and a specific monohydric alcohol, are excellent in hydrolytic stability, electrical insulating properties and miscibility with refrigerants, and also reduce the viscosity of lubricating oils for refrigerators, and these findings have led to the completion of the present invention.
Thus, the lubricating oil for refrigerators according to the present invention comprises at least one aliphatic saturated branched-chain carboxylic acid monoalkyl ester represented by the formula (1) 
wherein R1 is C1-C18 straight-chain alkyl or C3-C18 branched-chain alkyl; R2 is hydrogen, C1-C18 straight-chain alkyl or C3-C18 branched-chain alkyl; with the proviso that the total number of carbon atoms contained in the alkyls represented by R1 and R2 is 2 to 18 and that when R2 is hydrogen, R1 is a branched-chain alkyl; and R3 is C1-C20 straight-chain alkyl, C3-C20 branched-chain alkyl or C3-C10 cycloalkyl.
It has also been found that (I) at least one aliphatic saturated branched-chain carboxylic monoalkyl ester represented by the aforementioned formula (1) achieve(s) excellent advantages when used in combination with (II) a conventional lubricating oil for refrigerators, such as a fatty acid polyol ester, a phthalic acid ester, an alicyclic dicarboxylic acid ester, a polyvinyl ether, a hydrocarbon oil, a polyalkylene glycol or the like. For example, better hydrolytic stability is achieved in lubricating oils based on both components (I) and (II), and the reduction in the viscosity is achieved in lubricating oils based on components (I) and (II).
Thus, the present invention also provides a lubricating oil for refrigerators, characterized in that it comprises a mixture of
a) at least one aliphatic saturated branched-chain carboxylic acid monoalkyl ester represented by the formula (1) and
b) at least one member selected from the group consisting of
i) fatty acid polyol esters,
ii) phthalic acid esters,
iii)alicyclic dicarboxylic acid esters,
iv) polyvinyl ethers,
v) hydrocarbon oils, and
vi) polyalkylene glycols,
wherein the weight ratio of component a):component b) is 0.5:99.5 to 99.5:0.5.
It has also been found that the lubricating oil for refrigerators of the present invention, particularly its component, namely at least one aliphatic saturated branched-chain carboxylic acid monoalkyl ester represented by the aforementioned formula (1), has good miscibility with refrigerants, particularly hydrofluorocarbon refrigerants, and that a mixture of at least one such ester and a refrigerant, particularly a hydrofluorocarbon refrigerant, acts as a working fluid for refrigerators.
Accordingly, the present invention also provides a working fluid composition for refrigerators, comprising (I) at least one aliphatic saturated branched-chain carboxylic acid monoalkyl ester represented by the formula (1) of the present invention and (II) a refrigerant (particularly at least one hydrofluorocarbon refrigerant).
Additionally, the present invention provides a working fluid composition for refrigerators, comprising
(I) a) at least one aliphatic saturated branched-chain carboxylic acid monoalkyl ester represented by the above formula (1), and
b) at least one member selected from the group consisting of i) fatty acid polyol esters, ii) phthalic acid esters, iii) alicyclic dicarboxylic acid esters, iv) polyvinyl ethers, v) hydrocarbon oils and vi) polyalkylene glycols, and
(II) a refrigerant (particularly, at least one hydrofluorocarbon refrigerant).
Furthermore, the present invention provides a method for lubricating a refrigerator, particularly a method for lubricating refrigerators using at least one hydrofluorocarbon refrigerant, characterized by the use of the aforementioned lubricating oil for refrigerators.
Furthermore, the present invention provides use of an aliphatic saturated branched-chain carboxylic acid monoalkyl ester represented by the formula (1) as a lubricating oil for refrigerators, particularly as a luburicating oil for refrigerators using a refrigerant (particularly at least one hydrofluorocarbon refrigerant).
Similarly, the present invention provides use of a mixture of
a) at least one aliphatic saturated branched-chain carboxylic acid monoalkyl ester represented by the formula (1) and
b) at least one member selected from the group consisting of i) fatty acid polyol esters, ii) phthalic acid esters, iii) alicyclic dicarboxylic acid esters, iv) polyvinyl ethers, v) hydrocarbon oils and vi) polyalkylene glycols,
as a lubricating oil for refrigerators, particularly as a lubricating oil for refrigerators using a refrigerant (particularly at least one hydrofluorocarbon refrigerant).